The present invention relates to a graphic display controller for displaying characters, symbols, pictures, graphs, and the like on a display screen
A display control circuit for storing the level of brightness as "1" and "0" for each pixel of the screen according to the bit map refresh method is necessary to display characters, images, or graphs. There has been known a multiwindow display as means for enhancing the operability by simultaneously displaying a plurality of objects on a screen by use of the display control circuit. The multiwindow display is achieved by superimposing screen data or a plurality of rectangular subscreens having a variable size. The invention of the Japanese Patent Unexamined Publication No. 59-75338 accomplishes the multiwindow display by using two memory units and a direct memory access controller (to be abbreviated as DMAC herebelow) transferring data between the memory units. Although the invention is characterized by implementing a high-speed multiwindow display, the DMAC is so restricted that the data can be transferred in byte or word units, namely, data having a bit-unit boundary cannot be transferred. Furthermore, the CPU can access only the first memory, and the second memory and the DMAC are configured as a dedicated hardware, which can not be used for other than the processing executed by the CPU.
In the multiwindow display, the characters, images, or graphs need by positioned in bit units on the screen; and the DMAC usually comprises a plurality of channels and is hence desirably applied to a plurality of usages. The screen display data formed in the second memory not accessible from the CPU cannot be used by the CPU for other purposes (printing, for example); or the data cannot be stored in a temporary storage or cannot be read for the further processing.